4:9known by God. Their knowledge of God was not the result of their own inquiry, but the sovereign work of the One who opened their understanding. See “True Knowledge of God” at Jer. 9:24.

turn back again. Amazingly, Paul likens the bondage of ceremonial legalism to the bondage of pagan superstition. To accept circumcision as necessary for salvation is to turn back from the liberty of grace to the bondage of the world with its times and seasons (v. 10; Col. 2:8, 20–22), whether these times be Jewish or Gentile.

weak . . . principles. Before their conversion, the Galatians were subject to the “principles” of a pagan world: its false gods, its astrology, its seasonal rituals (v. 3 note).